1. Field of the Invention
The present invention relates to a switching power supply unit, and more particularly to a switching power supply unit of a self-excitation-type ringing choke converter (hereinafter abbreviated as RCC) system.
2. Description of the Related Art
In general, electronic equipment, such as electronic calculators or communication devices, needs a stable DC voltage. Thus, in order to supply a stable DC voltage to such electronic equipment from a commercial AC power supply, switching power supply units of the RCC system, in which relatively easy construction is possible to obtain high efficiency, have been widely used. A structure of such a switching power supply unit will be illustrated referring to FIG. 6.
In this figure, reference numeral 1 indicates a switching power supply unit, which has an input circuit 2, a main operation circuit 3, a voltage detection circuit 4, a control circuit 5, an output terminal OUT, and a ground terminal GND.
Among these components, the input circuit 2 has a rectifying diode bridge DB, a fuse F, and a filter circuit LF. Both the fuse F and the filter circuit are disposed between an AC power supply and an input end of the rectifying diode bridge DB.
In addition, the main operation circuit 3 has a smoothing capacitor C1 disposed between the output ends a and b of the diode bridge DB of the input circuit 2, a transformer T having a primary winding N1, a secondary winding N2 having the polarity opposite to that of the primary winding N1 and a feedback winding NB with the same polarity as that of the primary winding N1, an FET Q1 as a main switching element connected in series to an end of the primary winding N1 of the transformer T, a starting resistor R1 connected between the other end of the primary winding N1 and the gate as the control terminal of the FET Q1, a resistor R8 connected between the gate and source of the FET Q1, a rectifying diode D1 connected in series to an end of the secondary winding N2 of the transformer T, and a smoothing capacitor C4 connected between an end of the secondary winding N2 and the output terminal.
The voltage detection circuit 4, which is disposed on the output side of the main operation circuit 3, includes a resistor R5, a light-emitting diode PD on the light-emitting side of a photo coupler PC, a shunt regulator Sr, and resistors R6 and R7. Among these components, the resistor R5, the anode and cathode of the light-emitting diode PD, and the cathode of the shunt regulator Sr are mutually connected in series and are disposed in parallel to the capacitor C4 of the main operation circuit 3. In addition, the resistors R6 and R7 are also mutually connected in series and are also disposed in parallel to the capacitor C4. The node of the resistors R6 and R7 is connected to a reference of the shunt regulator Sr.
Furthermore, the control circuit 5 includes a resistor R9 and a capacitor C3 connected in series between one end of the feedback winding NB and the gate of the FET Q1, a transistor Q2 connected between the gate and source of the FET Q1, a resistor R2 connected between one end of the feedback winding NB and the base of the transistor Q2, a resistor R3 and a capacitor C2 connected in parallel between the base and emitter of the transistor Q2, a resistor R4, a diode D2, and a photo transistor PT on the light-receiving side of the photo coupler PC, which are mutually connected in series between one end of the feedback winding NB and the base of the transistor Q2.
Next, a description will be given of the operation of a switching power supply unit 1 having such a structure.
First, on startup, a voltage is applied to the gate of the FET Q1 via the resistor R1 to turn on the FET Q1. When the FET Q1 is turned on, a power supply voltage is applied to the primary winding N1 of the transformer T, and then, at the feedback winding NB is generated a voltage of the same direction as that of the voltage generated at the primary winding N1, whereby the FET Q1 is rapidly turned on by a positive feedback. In this situation, excitation energy is charged in the primary winding N1.
After that, when the base potential of the transistor Q2 reaches a threshold, the transistor Q2 is turned on, whereas the FET Q1 is turned off. This permits the excitation energy charged in the primary winding N1 of the transformer T during the ON time of the FET Q1 to be discharged as electric energy via the secondary winding N2. The energy is rectified by the diode D1 and is smoothed by the capacitor C4 to be supplied to a load.
When the excitation energy charged in the primary winding N1 of the transformer T is discharged via the secondary winding N2, a fly-back voltage VNB is generated at the feedback winding NB. Changes in the fly-back voltage VNB will be illustrated referring to FIG. 7. In this figure, at a point-in-time t11, the FET Q1 is turned cff and the fly-back voltage VNB is maintained at an almost fixed value to enter the so-called OFF time. Then, at a point-in-time t12, the voltage of the diode D1 becomes zero, and the fly-back voltage VNB begins to resonate. At a point-in-time t13, when the fly-back voltage VNB reaches a threshold Vth of the FET Q1, the FET Q1 is turned on. The dotted line of the fly-back voltage VNB indicates changes in a case where it is assumed that the fly-back voltage VNB continues to resonate even after the turn-on of the FET Q1. In this way, when the FET Q1 is turned on, a voltage is again applied to the primary winding N1 of the transformer T, whereby excitation energy is again charged in the transformer primary winding.
In the switching power supply unit 1, the above-described oscillating operation is repeated.
In a steady state, the output voltage of the load is divided by the resistors R6 and R7, and the divided detection voltage is compared with a reference voltage of the shunt regulator Sr. After the comparison, the amount of fluctuations in the output voltage is amplified by the shunt regulator Sr, and current flowing to the light-emitting diode PD of the photo coupler PC changes, in which the impedance of the photo transistor PT changes according to the light-emitting amount of the light-emitting diode PD. This operation permits the time for charging/discharging the capacitor C2 to be changed, so that the output voltage is regulated.
In the conventional switching power supply unit 1, however, switching losses under light load are large, which leads to reduced circuit efficiency.